Stem cell researcher Carolina Florian couldn’t believe what she was seeing. His old laboratory mice began to look younger. They were more lively and their fur was shinier. However, all he had done was a short treatment a few weeks earlier with a drug that modified the organization of proteins in a type of stem cell.

When technicians repeating Florian’s experiment in two other laboratories reached the same conclusion, Florian became more confident that the treatment in question would rejuvenate the animals. In two papers in 2020 and 2022, his team explained how this process extended the lifespan of mice and kept them in good physical condition into old age.

The purpose of Florian Elixir is the immune system. The immune cells he targeted are called hematopoietic stem cells, which give rise to mature immune cells. By circulating the blood, a mixture of these cells enters all the organs and affects all the functions of the body. However the molecular composition of hematopoietic stem cells changes during aging, and this upsets the balance of the immune cells that these stem cells produce.

Florian, who works at the Bleuge Biomedical Research Institute in Barcelona, ​​says reversing the misalignment that occurs over time appears to reverse many of the problems of aging, not only in the immune system but in the rest of the body as well.

In a paper published in March in the journal Nature, researchers show that restoring the balance between two key types of immune cells rejuvenates the immune system of aging mice and improves the animals’ ability to respond to vaccines and ward off viral infections.

Other scientists have used different experimental methods to reach a similar conclusion: Rejuvenating the immune system rejuvenates many organs in animals, at least in mice. More interestingly, evidence shows that aging of the immune system may cause aging of those organs.

The potential of the findings to help people stay healthy in old age is tantalizing. But applying this knowledge and using it in clinics will be challenging. Tampering with the immune system can be dangerous. Therefore, researchers initially aimed at low-risk goals such as improving the response of the elderly to vaccination and improving the efficacy of cancer immunotherapy.

Vittorio Sebastiano, a stem cell scientist at the Stanford School of Medicine in California, says the prospect that reversing aging might curb age-related diseases is enticing, but we proceed with caution.

The human immune system is a complex system whose many cellular and molecular components work together to help a person grow, protect him from infection, help heal wounds, and destroy cells that are becoming cancerous. But along with aging and changing the composition of the system, its efficiency decreases. In old age, people become susceptible to a wide range of infectious and non-infectious diseases and become more resistant to the protective power of vaccines.

Hematopoietic stem cells in the bone marrow produce both arms of the immune system. They differentiate into two main types (lymphoid cells and myeloid cells), which then undergo further differentiation.

Lymphoid cells are primarily responsible for adaptive immunity and include B cells that produce antibodies, T cells that help attack invaders and coordinate immune responses and natural killer cells that kill infectious cells. Myeloid cells comprise a group of cell types that are mainly involved in innate immunity.

One of the first changes in the immune system during aging is the shrinking of the thymus, which begins after puberty. The thymus is where T cells mature, but much of this tissue turns to fat by the third decade of life, reducing the production of new T cells and weakening the immune system.

In addition, the function of T cells changes with age and they are not as specialized in detecting infectious agents as before. The ratio of different types of immune cells in the circulation also changes. The ratio of myeloid to lymphoid cells is significantly skewed toward myeloid cells and this can cause inflammation. In addition, an increasing number of immune cells become senescent, meaning that they stop replicating but do not die.

Aging cells usually occur when they undergo mutations. When cells are in this condition, they begin to release inflammatory signals and mark themselves for destruction.

An important anti-cancer and wound-healing mechanism works best when young. But when too much damage accumulates with age and the immune cells themselves age, this mechanism is disrupted. Senescent immune cells, attracted by inflammatory signals from senescent tissues, secrete their own inflammatory molecules. Therefore, they are not cleared properly but instead, add to the inflammation that also damages the surrounding healthy tissues. This phenomenon is known as inflammatory aging. This turns into a terrible positive feedback loop, says Aran Akbar, an immunologist at University College London. Evidence shows that this feedback loop is initiated by the immune system.

Laura Niedernhofer from the University of Minnesota in Minneapolis has shown in a series of experiments in mice that the aging of immune cells causes the aging of other tissues. He says these cells are very dangerous.

His team used genetic methods to delete an important DNA repair enzyme in the immune system of mice. The animals remained healthy until adulthood, but after that, they were no longer able to correct the accumulated mutations, and different types of immune cells began to age.

A few months later, an increasing number of cells in organs such as the liver and kidney were also senescent, and signs of organ damage appeared. When the scientists gave old mice immune cells from the spleens of young, healthy mice, all of these effects were reversed. All of this suggests that modifying the aging properties of the immune system could help prevent or reduce age-related diseases, Niederenhofer says.

Many scientists are trying to do this from very different angles. Many approaches suggest that very short treatment of the immune system may have long-term effects and minimize side effects.

One of the ways to deal with aging immune cells is to use drugs to remove or inhibit the inflammatory factors that these cells release. Aging immune cells in humans can be changed, Niederenhofer says. If you smoke, they increase and if you exercise, they decrease.

Niederneuhofer is conducting a small clinical trial in elderly people with sepsis. Sepsis is a condition that becomes more deadly with age. His team is also conducting experiments to assess which types of immune cells are most involved in aging in the body, and their results could help design more precise treatments. Two types of cells (T cells and natural killer cells) are emerging as the main contenders, he says. He plans to examine natural products and approved drugs for their ability to interact with these types of immune cells during aging.

Akbar thinks targeting inflammation may be just as effective as targeting senescent cells. He and his colleagues conducted a study in healthy volunteers using the investigational compound lozepimod, which inhibits an enzyme involved in the production of a type of inflammatory molecule called cytokines. They treated volunteers with this drug for four days and then measured their skin’s response to an injection of the chickenpox virus over the course of a week. Most people are exposed to this virus during their life and this virus often stays in the body.

As people age, they lose their immunity to the chicken pox virus, and this time it can appear as shingles. The drug restored the immune response in the skin of older volunteers to a level similar to that of young volunteers. Akbar has found in unpublished studies that the same strong results persist up to three months later. Temporarily inhibiting inflammation in this way to keep the immune system functioning may similarly enhance the response of older patients to flu vaccinations, he says.

The value of priming the elderly immune system prior to vaccination has been demonstrated in a series of clinical trials led by Joanne Mannick, CEO of Boston, Massachusetts-based Tornado Therapeutics. The trials tested analogs of the drug rapamycin and other drugs with similar mechanisms that target the immune system and are approved to prevent organ transplant rejection and to treat certain cancers.

The mentioned drugs inhibit an enzyme called mTOR, which is vital for many physiological functions and whose function is impaired in aging. Participants were treated with doses of the drug that were low enough to avoid side effects for several weeks before receiving the flu vaccine. This treatment regimen improved their response to the vaccine and increased their immune system’s ability to resist viral infections.

However the drug rapamycin can increase susceptibility to infection and affect metabolism, so Manick is planning trials with similar drugs that could be safer. “There are different ways to improve the immune system,” he notes.

Another way is to try to restore thymus function to maintain the production of new T cells. Jarrod Dudakoff, an immunologist at the Fred Hutchinson Cancer Center in Seattle, is studying the basic biology of thymus cells to understand how they regenerate after bouts of stress. Dudakov says it’s a little early to see how our understanding of this can be applied in the clinic. But he thinks it’s important to preserve the ability of the thymus to produce T cells.

Others try to fight aging by producing thymus tissue from powerful stem cells and then transplanting it. But Greg Fahey, chief scientific officer at Intervene Immune in Torrance, Calif., says there’s no need to wait to achieve those long-term prospects because synthetic growth hormone regenerates thymus tissue. He is conducting small studies in healthy volunteers using growth hormones as part of a mixture of compounds.

Preliminary results show that the amount of functional thymus tissue in the participants increased and their epigenetic clock (a biomarker of aging) was reversed by several years. Fahey is conducting further testing to see if the drug combination also improves the physical condition of the participants.

Another approach that has yet to reach the clinic is reprogramming immune cells to try to turn back the clock on cells that have aged. This procedure involves temporarily placing the cells in a dish exposed to a combination of transcription factors that induce a pluripotent state in mature cells.

Sebastiano and colleagues have shown in human cells that this corrects the epigenetic changes that accompany aging. He has founded a startup to use this technique to tackle a type of cancer treatment called CAR T, in which T cells are engineered outside the body to target and destroy a person’s cancer. However, the T cells may age before they are returned to the person. Rejuvenating them makes production faster and more powerful, says Sebastiano.

In his studies, Florian used a four-day treatment with a compound called CASIN, which inhibited part of this process to correct polarization and help the mice live longer. When hematopoietic stem cells from aged mice that had received CASIN were transplanted into aged mice that had not received the treatment, the same life-extending effects were seen. Florian hopes to turn his results into a practical method in the clinic. He thinks his drug may help rebuild the immune system after receiving cancer chemotherapy.

Research on immune aging faces major challenges. One of the challenges in aging studies of all organs is the inability to measure aging accurately. “We don’t know in a quantitative, measurable, predictable way what aging means at the molecular level in different cell types,” says Sebastiano. “Without these metrics, it is very difficult to demonstrate rejuvenation.”

Another challenge is the difficulty in determining the characteristics that make an immune cell unique. Until recently, it was difficult to show where each of the immune cell subsets lived and how they changed over time. But technologies such as single-cell RNA sequencing, which quantitatively measures genes expressed in single cells, have made the analysis more challenging. For example, a large study of immune cells in the blood of humans and mice across a range of ages, published last November, revealed 55 subpopulations. Only 12 subpopulations of cells changed with age.

By collaborating with different research areas, scientists hope to prove that the immune system plays an important role in healthy aging. Don’t expect an elixir of youth anytime soon, says Florian. Aging research will take a long time, but it can help design tools that will be transformative.